Method for isomerizing glucose syrups

ABSTRACT

The present invention is directed to a method for enzymatically isomerizing glucose in glucose-containing liquors. The presence of relatively small amounts of water-soluble salts of sulfurous acid during the enzymatic isomerization of glucose in glucosecontaining liquors reduces color formation therein and increases the stability of the glucose-isomerizing enzyme.

States Patent Inventors William P. Cotter;

Norman E. Lloyd; Charles W. l'linman, all of Clinton, Iowa Appl. No. 765,654

Filed Oct. 7, I968 Patented Nov. 30, 1971 Assignee Standard Brands Incorporated New York, N.Y.

METHOD FOR lSOMERlZlNG GLUCOSE SYRUPS 9 Claims, No Drawings [56] References Cited FOREIGN PATENTS 1,103,394 2/1968 Great Britain l95/3l OTHER REFERENCES Pollard, Chemical Abstracts Vol. 57, 0 l 03 l 3f Pieck et al., Chemical Abstracts Vol. 60, 09462h Primary Examiner-A. Louis Monacell Assistant Examiner-Gary M. Nath Attorney-Aaron B. Karas ABSTRACT: The present invention is directed to a method for enzymatically isomerizing glucose in glucose-containing liquors. The presence of relatively small amounts of watersoluble salts of sulfurous acid during the enzymatic isomerizw tion of glucose in glucose-containing liquors reduces color formation therein and increases the stability of the glucoseisomerizing enzvme.

3 METHOD FOR ISOMERIZING GLUCOSE SYRUPS THE INVENTION The present invention relates to a process of enzymatically isomerizing glucose in glucose-containing liquors. More particularly, the invention relates to a process of enzymatically isomerizing glucose in glucose-containing liquors whereby color development in theliquors during isomerization is reduced.

The major use of glucose and of corn syrups containing glucose is in food processing, for example in the baking, beverage, canning and confectionery industries, to provide sweetness, body or to regulate crystal growth. However, because glucose inherently lacks a high degree of sweetness and has a relatively bland flavor, its uses are somewhat limited. This is overcome, to some extent, by mixing glucose or corn syrups with sucrose or invert syrups to enhance total sweetness. This has not proven entirely satisfactory, however, because of economic and other'factors involved. It has been recognized that if during the production of corn syrups and other glucose-containing syrups a significant proportion of the glucose could be converted to fructose, syrups would be provided that are sweet enough to satisfy additional purposes.

It has long been known in the art that glucose can be converted to fructose by heating a glucose-containing liquor, such as a corn syrup, in the presence of an alkaline catalyst. Because of the nonselectivity of alkaline catalysts various objectionable byproducts are produced, such as large amounts of colored bodies and acidic materials. To refine the alkaline isomerized liquor to remove the objectionable byproducts thereof, requires rather complicated and costly refining procedures. Consequently alkaline isomerization, as far as we know, has not been practiced commercially, due probably, to the economics involved with refining the alkaline isomerized liquor and the relatively poor quality of the resulting product.

Various micro-organsims produce enzymes which isomerize glucose in glucose-containing syrups to fructose. These enzymes are referred to in theart as glucose isomerase. Ari article appearing in Science, Vol. 125, pp. 648-9 (1957) discloses that an enzyme derived from Pseudomonas hydrophila will isomerize glucose to fructose. Also British Pat. No. 1,103,394 and Japanese Pat. No. 17,640 (1966) disclose that micro-organisms classified as belonging to the Srreptomyces genus, such as Streplomycesflavovirens, Streptomyces achromogenes, Slreptomyces echinatus, Streplomyces albus, and Streptamyces phaeochromogener produce glucose isomerase.

Although glucose-isomerizing enzymes are more selective in converting glucose to fructose than is an alkaline catalyst, there are still a number of problems associated with the commercial use of these enzymes. For example, appreciable quantities of colored bodies are produced during enzymatic isomerization, which make the resulting products difficult to refine. Also there is a tendency for the isomerizing enzyme to be inactivated in a shorter period than is desired. The formation of colored bodies and the inactivation of the isomerizing enzyme are largely dependent upon the conditions under which the isomerization reaction is carried out. If the reaction is performed for relatively long periods of time and/or at high temperatures, in order to obtain high yields of fructose, there will be greater amounts of colored bodies formed and the enzyme will be inactivated to a greater degree.

The purity of the glucose isomerase preparation also affects the formation of colored bodies in the isomerized liquor. lf relatively large amounts of extraneous materials are present in the glucose isomerase preparation, there is a greater tendency for larger amounts of colored bodies to be formed.

It is the principal object of the present invention to provide an enzymatic method of isomerizing glucose in glucose-containing liquors whereby the formation of colored bodies in the liquors during isomerization is reduced.

This object, and other objects of the present invention which will be apparent from the following description, are attained by providing a glucose-isomerizing enzyme in a glucose-containing liquor and subjecting the liquor to isomerizing conditions, there being present in the glucose-containing liquor during isomerization a small amount of a water-soluble salt of sulfurous acid sufiicient to measurably reduce the formation of colored bodies below that level obtained by carrying out the enzymatic isomerization without the presence of the water-soluble salt of sulfurous acid.

Although the present method does not completely eliminate the formation of colored bodies, hereinafter referred to as color, during isomerization, the small amount of color which is produced may be removed by relatively simple refining procedures.

In the process of the present invention, the salts of sulfurous acid may be provided in the glucose-containing liquors by any convenient method. For instance, sulfite or bisulfite salts, or other substances which will generate sulfite or bisulfite ions, e.g., S0, or H 50 solution, may be incorporated directly into the glucose-containing liquor before the isomerization process is carried out or may be incorporated into the liquor during isomerization. Also sulfite or bisulfite ions may be provided in the glucose-containing liquors by passing such liquors through ion exchange resins in the sulfite form. Preferably, however, the bisulfite and sulfite salts are provided in the glucose-containing liquors before the isomerization process is initiated since the full benefit of the presence of these salts will thereby be obtained.

The preferred microorganisms used to produce glucose isomerase for use in the present isomerization process are those belonging to the Streptomyces genus. The most preferred micro-organism is Streptomyces sp. ATCC 21175. The taxonomical characteristics of this micro-organism are shown below.

TAXONOMICAL CHARACTERISTlCS OF STREPTOMYCES SP. ATCC 2] A. Morphological Observations 1. Spiral sporophores making 3 to 6 turns; a few incompleted sporophores forming hooks and loops.

Culture Medium B. Color of Colony The most representative color of the spores and aerial mycelia en masse" on the surface of mature colonies is beige brown or mist brown, matching color tab 3 ig on the Tresner-Backus color wheel. C. Reverse Side of Colony No distinctive pigmentation. Gray or brownish yellow on yeast extract-malt extract agar, oatmeal agar and starch agar. D. Color in Medium No pigment formed. E. Carbon Utilization L-arabinose, D-fructose, i-inositol, D-mannitol, rhamnose, and D-xylose are utilized for growth. No growth on sucrose and raffinose. F. Other Physiological Properties Growth is strictly aerobic, mesophilic. No growth at 50 C.

on yeast extract-malt extract agar. Proteolytic activitypositive on Gordon and Smith casein agar. Diastatic activity-positive on inorganic salts-starch agar. Volatile compounds with earthy or moldy odor are produced during the active growth of the culture on most media.

Another preferred micro-organism belonging to the Streptomyces genus used to produce glucose isomerase is Streptomyces sp. ATCC 2l l76.

Since glucose isomerase is primarily produced intracellularly by these micro-organisms, a source of glucose isomerase may be provided by simply harvesting the cells from the growth media. The glucose isomerase may be separated from the cells of these micro-organisms by techniques known in the art, i.e., sonic treatment, etc., and used to isomerize glucose in a glucose-containing liquor to fructose or the cellular material may be used directly. When cellular material is used there is the tendency for more color and other objectionable byproducts to be produced because of the extraneous materials which are present along with the glucose isomerase, than when separated and purified glucose isomerase is used. However, the techniques necessary to separate the glucose isomerase are generally time consuming and involve added expense. Because of this the present process is particularly applicable to suppression of color fonnation when an enzymatic isomerization reaction is carried out using as a source of the glucose isomerase cellular material. Since, generally during the enzymatic isomerization there is required as enzyme activators, salts of magnesium, cobalt, chromium, and/or manganese, these salts of sulfurous acid are preferred. In the case of Streptomyces sp. ATCC 21 175 the preferred salt is magnesium sulfite.

The preferred pH range for performing the enzymatic isomerization reaction is from about 6.0 to about 8.5 with a pH range of from about 6.5 to about 7.5 being most preferred. The temperature of the glucose-containing liquor during isomerization may vary widely, although it is preferred that the glucose-containing liquor be at a temperature of from about 45 to about 80 C. during the isomerization reaction, and most preferably be at a temperature of from about 50 to about 65 C.

The amount of bisulfite or sulfite salts provided in the glucose-containing liquor may vary, but under the preferred conditions of the present invention sufficient amounts of these salts are added to provide an SO content in the liquor of from about 0.02 to about 0.3 percent by weight based on the dry substance content of the liquor, and most preferably from about 0.03 to about 0.07 percent by weight on the same weight basis. Although at greater concentrations of SO, there will be a relatively long period during the isomerization reaction when less color is produced, than in the case of an isomerization reaction without the presence of sulfites or bisulfites, after this initial period the rate of color formation will increase very rapidly until the color formed will exceed that formed when the isomerization reaction is carried out without the presence of sulfites or bisulfites. Therefore when these salts are used, the isomerization reaction should be terminated before the color formed reaches a point where the subsequent removal thereof is difl'rcult.

Although the glucose-isomerizing enzyme is relatively stable at high temperatures it is subject to thermal denaturation nonnal to all proteins. The presence of the sulfite salts during the isomerization reaction, especially at high isomerization temperatures, surprisingly exerts a protective effect towards the glucose-isomerizing enzyme. This provides the benefit that lesser quantities of the enzyme are needed to achieve the same yield of fructose when sulfites or bisulfltes are present, or conversely for the same quantity of enzyme, higher yields of fructose can be obtained.

In order to more clearly describe the nature of the present invention specific examples will hereinafter be described. It should be understood, however, that this is done solely by way of example and is intended neither to delineate the scope of the invention nor limit the ambit of the appended claims. In the examples and throughout the specification, percentage refers to percent by weight and is based on the dry substance weight of the glucose-containing liquor unless otherwise specified.

The analytical methods referred to in the following examples were performed as follows:

The color of the glucose-containing liquor was detennined spectrophotometrically by measuring the absorbance at 450 my. and 600 mp. of an appropriately diluted liquor in a 1 cm. cell versus water as a reference. The spectrophotometer was a Beckman DK-2A, manufactured by Beckman Instrument Co. The color was calculated by using the following formula:

Color=L A =absorbance at 450 mp. A =absorbance at 600 my. C =concentration in grams of dry substance per 100 ml. of liquor.

SO: CONCENTRATION IN THE GLUCOSE-CONTAINING LIQUOR Sulfur dioxide in the liquors was determined as follows: A sample of the liquor in the range of 50-60 g. was weighed accurately into a dish and transferred quantitatively into an 800 ml. Kjeldahl flask employing 300 ml. of distilled water. Ten ml. of concentrated phosphoric acid was added followed by l g. of sodium bicarbonate. The flask was immediately connected to a standard Kjeldahl distillation apparatus and approximately 250ml. distilled into a Erlenmeyer flask containing 25 ml. of water and 10-12 ml. of 0.8 percent sodium hydroxide solution. When the distillation was complete, the distillate was acidified with phosphoric acid and 2 ml. of starch paste indicator added. The solution was then titrated with 0.0625N iodine solution (1 ml. equivalent to 0.002 g. of 80,) until a blue color persisted for 1 minute. Percent SO, dry basis was calculated as follows:

titre(ml.) 0.002 Sample wt. g.) X Dry Substance (percent) Percent SO Fructose content of the isomerized liquor was determined by measuring the change in specific rotation which occurred during isomerization. Specific rotations were measured using a Bendix Corporation NPL Model 969 Automatic Polarimeter. The rotations were determined at a concentration of 5 g./l00 ml. in a glass cell thermostated at 25 C. Path of the cell was 20 mm. The specific rotations were determined at the beginning of the isomerization reactions after all ingredients in the isomerization reaction mixtures had been combined. To determine change in fructose content the specific rotation of the isomerized liquor at time t was determined. All samples were adjusted to pH 4.0 with dilute hydorchloric acid in order to halt enzyme action before dilution for determination of rotations. Change in fructose content was calculated by using the following formula:

or specific rotation at start of isomerization a, specific rotation at time t In the formula the factor l38.9 is the change in specific rotation which occurs when glucose is converted completely to fructose.

Percent F GLUCOSE ISOMERASE ACT IVIT Y was determined at pH 7.5 instead of 7.2. Thus the definition of a glucose isomerase unit (GIU) is that amount of enzyme which under the test conditions (pH 7.5, 70 C., 1 hour, test solution 0.1M in D-glucose, 0.005M in magnesium sulfate, and 0.05M in pH 7.5 phosphate buffer) will produce 1 mg. of

D-fructose per hour. Fresh cells and dry cells were suspended in distilled water and sonicated with a Branson Model S75 sonifier for 2-3 minutes in order to destroy the cell structure and release the enzyme into the liquid phase. The sonicates EXAMPLE 11 This example illustrates the enzymatic isomerization of glucose in glucose-containing liquors using various amounts of were centrifuged and appropriate aliquots of the clear su- 5 isomerase in the Presence of various amounts of pernate taken and diluted to the proper range (0-20 GlU/ml.) l for assay by the automated method Two senes of four glucose-containing ilClllOl samples (mother liquor from primary dextrose cyrstallization, 90DE) EXAMPLE] were prepared containing 0.005M magnesium chloride and t 0.00lM cobalt chloride. Series A contained 53.4 percent dry i example luustfaies h CRZYmaUC lsomenzatlon of substance, and Series B contained 56.7 percent dry substance. cose m glucose'coilmmmg liquors the Presence and absence To the samples, were added various quantities of the air-dried Ofsulfiie filter cake of example I and sulfite salts. The isomerizations stleptomyces P- {\TCC 11 was grown f aemblc were carried out at 70 C. for various times under an at- '8 fermentation colfdltlons 3 P a p mosphere of nitrogen. The color and fructose formed during 126d aqueous medlllm conmlmng 1 Percent Sofbltol, P the isomerization reactions were determined and are shown cent dextrose, sufficient corncob hydrolysate to provide 1 perb l i bl 11], cent xylose, 4 percent steep water at 29 B. and 0.024 percent TABLE III cobaltous ion. The fermentation was carried out at 30 C., an airflow of l volume of air per volume of medium per minute 3333; and a back pressure of 10 p.s.i. The fermenting broth was Percent; Percent Total time, P r nt mechanically stirred at 200 r.p.m. and after 65 hours 4 per- Sample Nags); NaHSOa S01 fructose cent filter aid was admixed into the broth and the cellular SeriesA (4.2 GIU/g. of dry substance) material harvested from the broth by filtration with suction. 0 0 The filter cake was washed with demineralized water, broken 17 12. 9 ii into small pieces and dried for 5 hours in a forced-air oven at 1 0 05 0 025 2? 3g 33 an air temperature of 140 F. The activity of the air-dried filter 66 31. 9 157 cake was 660 GlU/g. 22 22:: 2% A series of four glucose-containing liquors prepared from 0 9 hydrolysates of cornstarch were prepared having the composi- 5:}; g trons shown in the following table: 2 0. 05 0. 05 0. 054 41 26. 9 s4 66 2 -2 32 00 6. TABLE I 114 36. g 647 0 17 14. 4 4 Sample 29 20.8 7 3 0.075 0. 075 0.081 41 26. 4 17 66 31.8 291 2 3 4 90 35.8 511 114 35.8 645 0 0 Glucose content (percent g g dry basis 55.5 55.5 55.5 67- 4 v 0. 10 0 0 0 03 4i 2 2 6 CoCl,-6H,O (molarity) 0.001 0.001 0.001 0.0m 66 7 130 Na,S0, (percent dry basis) 0.25 90 36. 3 470 51 0,6110 (molarity) 0.005 0.005 114 4 iijfi (pemcm dry 0 25 0 25 Series B (8.5 GIU/g. of dry substance) Total SOI (percent dry 0 0 6 basis) 0. l 3 0. l 2 none 0.12 251 g Glucose lsomerasc (Gllg. 1 0 05 n 0 025 dry basis) 2.3 2.3 2.: 4.6 22 g g: 92 40. 5 550 0 0 5 These samples were isomerized at a temperature of 70 C. 38-2 3 for 92 hours with the pH thereof being maintained at 6.5 by 2 Q05 38 34.7 58 the addition of a 0.5-percent solution of sodium hydroxide. An 8; 23: g 2%; atmosphere of nitrogen was maintained over the three samples 0 0 4 which contained the sulfites. The color and the fructose con- 5 5 tent of the liquors were determined throughout the isomeriza- 3 1075 01175 081 38 35.8 24 tion. The results of these determinations are shown in table ll. 8; 5 13 33? v TABLE II Sample Number Isomerizatlon Percent Percent Percent Percent time (hours) fructose Color fructose Color fructose Color fructose Color As seen from table II, as the amount of fructose increased 2 20 g g the color of the isomerized liquor also increased. In each of 1 5 7 the isomerization reactions carried out in the presence of 4 08 l5 sulfites less color was formed than in the liquor which con- 92 6 tained no sulfites on an equal fructose formed basis. Also it is seen that more fructose formed in the samples containing the sulfite salts indicating that the sulfites reduced the degree of inactivation of the enzyme during the isomerization reaction.

From table Ill, it is apparent that generally at comparable fructose levels increasing sulfite content resulted in less color being produced. Also, the user of higher levels of glucose isomerase results in lower colors at comparable fructose and sulfite levels.

suspending in water and centrifuging. This water solution containing the enzyme was dialyzed continuously against l gallons of demineralized water at 3 C. The dialysate was concentrated to 136 g. and was then lyophilized to obtain 15.9 g. of

EXAMPLE 5 purified glucose isomerase preparation having an activity of This example illustrates the use of ion exchange resins to provide sulfite ions in a glucose-containing liquor and the en- T mxmres were prepared havmg the zymatic isomerization of the glucose-containing liquor. lowmg composmon:

A glucose-containing liquor (mother liquor from primary 3.0M glucose dextrose crystallization, 90DE) containing 60 g. dry substance per 100 ml. and having a color of 8 was heated to 70 C. and o-oolM cobalt chlonde sufficient magnesium chloride and cobalt chloride added to O 005M h] provide a molar concentration therein of 0.005 and 0.001, magneslumc on C respectively. One-tenth of 1 percent sodium bisulfite was Sufficient purified glucose isomerase preparation was added added and the pH of the liquor was adjusted to 6.5 with dilute to provide 1 1.4 GlU/g. glucose. To one of these isomerization sodium hydroxide. A sufficient amount of dried filter cake of mixtures was added enough sodium bisulfate to make it Streptomycer sp. ATCC 21 175 prepared according to example 0.005M in respect to this salt (0.096 percent S0 The mixwas added to provide 9.0 GlU/g. dry substance. The liquor tures were maintained under nitrogen atmosphere at pH 6.5 was isomerized for 24 hours at a temperature of 70 C., and and 70 C. Aliquots were removed at the start of the isomethe pH during the isomerization was maintained at 6.5 by the rization and after 20, 44 and 92 hours the fructose and color addition of a dilute sodium hydroxide solution. The fructose determined. The results of these determinations are shown in content and the color were determined after 22 hours and table V. were 33.8 percent and 18, respectively. After 24 hours the Also shown in table V are residual glucose isomerase activiisomerized liquor was filtered and divided into four 400 ml. ties at the various sampling times. The residual enzyme activiportions each of which contained 233 g. dry substance. Each 25 ties in Table V were determined as follows: portion was passed separately through ion exchange columns A test solution was prepared by mixing 25 ml. of isomerate containing various amounts of Dowex ll resin (manufactured with 25 ml. of a stock solution which was 3M in glucose, by Dow Chemical Co.) in the sulfite fonn. After ion exchange 0.2M in pH 6.5 sodium maleate buffer, 0.02M in magtreatment, the pH of the portions was adjusted to 6.5 with a nesium sulfate, and 0.001M in cobalt chloride. The test dilute solution of sodium hydroxide and sufficient cobalt solution was placed in a water-jacketed polarimeter cell chloride added to give a molar concentration of 0.005. The (20 mm. path). Hot water was circulated through the temperature of the portions was maintained at 70 C. and at a jacket to maintain the contents of the cell at 70 C. The pH of 6.5 by adding during the isomerization a dilute sodium cell was placed in a Bendix Automatic polarimeter hydroxide solution. The color and the fructose content during equipped with a recorder and the rate of change in optical the isomerization reaction were determined and are shown in rotation determined. From the rate of change in optical table lV. rotation, the rate of formation of fructose (V,) catalyzed TABLE IV Cubic feet 0! pound of dry resin per pound of dry Isomerizasubstance tion time, processed Percent Sinnplc Sample description hours (X10 fructose Color Initial isomerization g ::::::::::::"'5 g 24 25. 0 35.8 7 1 Alter treatment w/ion exchange resin 72 25. 0 42.8 63 24 12. 5 35.8 9 2 a: a2 2&2 $8 72 8.3 43. 7 234 24 6.3 35. 6 9 4 EXAMPLE IV by the residual glucose isomerase was calculated. The This example illustrates the stabilizing effect of sulfite ions 22:22:13 3 3: i gggg gsig gg on the glucose-isomenzing enzyme under the conditions of an to the following equation. isomerization reaction.

475 g. of dried filter cake prepared as in example I was V (K.+C F(1 K,/K ))(C 0 suspended in a sufficient amount of 0.005M cobalt chloride 65 E/Ci: CBkf(C +1/ C solution to obtain 5 liters. The suspension was adjusted to pH 6.25 and 58 C. and maintained with stirring at these condi- V,= rate of fructose formation in moles liter" hr. tions for 6 hours. The suspension was then cooled to room C =total concentration of glucose and fructose in moles per temperature and filtered to obtain a cell-free extract. The cellliter. free extract was concentrated tenfold using a Rinco rotary F =concentration of fructose in moles per liter. evaporator. 383 g. of the concentrated cell-free extract was C =concentration of dry substance (g./ml.) in test solution. placed in a breaker and the temperature lowered to 1 C., and k, pseudo-first-order rate constant for the breakdown of 255 g. of acetone added with stirring to form a precipitate. enzyme-glucose complex to enzyme plus fructose (equal After 15 minutes, the precipitate was removed by centrifugato 0.012 moles fructose liter" hr GlU at pH 6.5 and tion at 2,000 rpm. and was extracted twice with water by K, Michael is constant for substrate 0.580M glucose at pH 6.5 and 70 C.).

K p Michaelis constant for product (0.936M fructose at pH 6.5 and 70 C.).

K apparent equilibrium constant for the reaction (1.094

at 70 C.

C; concentration of dry substance (g./ml.) in isomerate.

C, concentration of dry substance (g./ml.) in stock solution.

Referring to table V it is seen that of the 20 GlU/g. added to the isomerates, 99 and 98 percent were recovered in the hour samples as measured by the above technique. At the end of 80 hours the isomerate containing the sulfite salt retained 29 percent of the original enzyme activity whereas the sample containing no sulfite had only 13 percent residual activity.

l. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose comprising providing a glucose-isomerizing enzyme in a glucose-containing liquor and subjecting the liquor to isomerizing conditions, there being present in the glucose-containing liquor during isomerization a small amount of a water-soluble salt of sulfurous acid suffrcient to measurably reduce the formation of color bodies below that level obtained by carrying out the enzymatic isomerization without the presence of the water-soluble salt of sulfurous acid.

2. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose as defined in claim 1,

wherein the amount of a water-soluble salt of sulfurous acid provided in the glucose'containing liquor during isomerization is sufiicient to provide a level of SO, in the liquor of from about 0.02 to about 0.3 percent by weight based on the dry substance content of the liquor.

3. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose as defined in claim 2, wherein the amount of a water-soluble salt of sulfurous acid provided in the glucose-containing liquor during isomerization is sufficient to provide a level of SO, in the liquor of from about 0.03 to 0.08 percent by weight based on the dry substance content of the liquor.

4. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose as defined in claim 2, wherein the glucose-isomerizing enzyme is produced from a micro-organism of the Streptomyces genus.

5. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose as defined in claim 4, wherein cellular material containing glucose-isomerizing enzyme is provided in the liquor.

6. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose as defined in claim 5, wherein the pH of the glucose-containing liquor during isomerization is maintained at a value from about 6.5 to about 7.5.

7. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose as defined in claim 6, wherein the temperature of the glucose-containing liquor during isomerization is maintained at a level of from about 50 to about 65 C.

8. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose as defined in claim 7, wherein the glucose-isomerizing enzyme is produced from Streptomyces sp. ATCC 21175 or Streptomyces ATCC 21176.

9. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose-as defined in claim 8, wherein the water-soluble salt of sulfurous acid provided in the glucose-containing liquor during isomerization is selected from the group consisting of magnesium sulfite, magnesium bisulfite and mixtures thereof. 

2. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose as defined in claim 1, wherein the amount of a water-soluble salt of sulfurous acid provided in the glucose-containing liquor during isomerization is sufficient to provide a level of SO2 in the liquor of from about 0.02 to about 0.3 percent by weight based on the dry substance content of the liquor.
 3. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose as defined in claim 2, wherein the amount of a water-soluble salt of sulfurous acid provided in the glucose-containing liquor during isomerization is sufficient to provide a level of SO2 in the liquor of from about 0.03 to 0.08 percent by weight based on the dry substance content of the liquor.
 4. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose as defined in claim 2, wherein the glucose-isomerizing enzyme is produced from a micro-organism of the Streptomyces genus.
 5. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose as defined in claim 4, wherein cellular material containing glucose-isomerizing enzyme is provided in the liquor.
 6. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose as defined in claim 5, wherein the pH of the glucose-containing liquor during isomerization is maintained at a value from about 6.5 to about 7.5.
 7. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose as defined in claim 6, wherein the temperature of the glucose-containing liquor during isomerization is maintained at a level of from about 50* to about 65* C.
 8. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose as defined in claim 7, wherein the glucose-isomerizing enzyme is produced from Streptomyces sp. ATCC 21175 or Streptomyces ATCC
 21176. 9. A process for enzymatically isomerizing glucose in a glucose-containing liquor to fructose as defined in claim 8, wherein the water-soluble salt of sulfurous acid provided in the glucose-containing liquor during isomerizaTion is selected from the group consisting of magnesium sulfite, magnesium bisulfite and mixtures thereof. 